Transistor-transistor logic (TTL) is one of the prevalent types of bipolar logic employed in integrated circuits. Although the definition of TTL has become blurred over the years due to the practice of merging TTL with other types of bipolar logic while still terming the result "TTL", TTL continues to be a "ground-referenced" logic. A TTL device typically changes state when the input voltage passes 2V.sub.BE with respect to ground reference (0 volt). V.sub.BE is the standard voltage of about 0.7-0.8 volt across the base-emitter junction of an NPN transistor when it is just conductive.
Referring to FIG. 1, it illustrates the basic features of a conventional "TTL" inverter. The operation of this device is very well known. For example, see U.S. Pat. Nos. 3,824,408 and 4,321,490. Accordingly, only a few pertinent comments will be made about it.
All of the transistors shown in FIG. 1 are NPN devices. Certain of the transistors are Schottky clamped to prevent the so clamped transistors from going into deep saturation. An NPN transistor is Schottky clamped by connecting a Schottky diode between its base and collector as indicated in FIG. 2. Returning to FIG. 1, power is supplied to resistors RA, RB, and RC from a source of a supply voltage V.sub.CC which is at a suitable high level relative to ground.
Assume that input voltage V.sub.IN at the inverter input terminal is initially at a low value less than 2V.sub.BE above ground. PN input diode DA is turned on so that input transistor QA, phase-splitting transistor QB, and low-level drive transistor QC are all turned off. The high-level driver is turned on to pull output voltage V.sub.OUT up to a logical "1" somewhat below V.sub.CC.
When input V.sub.IN is increased, the voltage at node A between the DA anode and the QA base also increases. Transistors QA, QB, and QC turn on when V.sub.IN reaches about 2V.sub.BE above ground. Due to the 1V.sub.BE across the base-emitter junction of each of transistors QB and QC, they clamp the QA emitter voltage at 2V.sub.BE above ground. QA then similarly clamps node A at 3V.sub.BE above ground. A further increase in V.sub.IN causes diode DA and the high-level driver to turn off. Transistor QC pulls output V.sub.OUT to a logical "0" near ground. The reverse events occur when V.sub.IN is subsequently returned to a low value. If V.sub.CC is 5 volts, the V.sub.OUT swing is usually about 3 volts.
The output signal of a TTL gate is often supplied to a circuit employing another type of "ground-referenced" logic. In some cases, the TTL output voltage swing is too great for the non-TTL circuit. If the TTL output drives a circuit using a non-saturating logic such as ground-referenced emitter-coupled logic or current-tree logic, a relatively high TTL output swing can force the normally non-saturating transistors into saturation. The result is a loss in speed, which may be accompanied by a loss in data and/or excessive power dissipation. It would be desirable to have a simple device that can make rapid translations from the output voltage levels of a TTL circuit to a "ground-referenced" voltage level at a lesser voltage swing.